EFTs in the US — I don't know if this type of security exists elsewhere — have some characteristics of both stocks and of mutual funds.
For example, some mutual funds that exist in various classes (retail investor, institutional, adviser, etc.) are available as an ETF that is considered, at least by the mutual fund company and perhaps regulators, as a class of mutual fund. And like mutual funds, EFTs are a basket of other investments, e.g., stocks or bonds, or both.
But to me it makes more sense to enter them into Gnucash as "stock" instruments if only because they trade on a stock exchange (mostly the Amex, I believe) like stocks. Yes, some EFTs are full of bonds, but bonds, too, but unless you are tracking your asset allocation, which is another issue entirely, it doesn't matter.
In the end, however ... and I say this as a Gnucash noob ... I don't think it makes any difference. Or at least I haven't (yet) seen why it would make a difference. And if I am wrong, I hope someone will explain why. Thanks.